1. Field of the Invention
The invention relates to an image forming apparatus such as a flat type image display apparatus or the like using electron-emitting devices and light emitting members and, more particularly, to the invention having a feature in a spacer which is interposed between an electron source substrate on which the electron-emitting devices are formed and a substrate having the light emitting-members in order to keep a distance between both of those substrates.
2. Related Background Art
Hitherto, as for an image display apparatus including a CRT, a further larger display screen is demanded and it is an important subject to realize a thin size and a light weight of the apparatus in association with the realization of the large display screen. As an image display apparatus which can realize the thin size and light weight, the applicant of the present invention has proposed the flat type image display apparatus using surface conduction electron-emitting devices. According to such an image display apparatus using the surface conduction electron-emitting devices, a rear plate having a plurality of electron emitting devices and a face plate having light emitting members and anode electrodes are sealed through a frame member, thereby forming a vacuum container. In such an image display apparatus, to prevent deformation and destruction of the substrates due to an atmospheric pressure difference between the inside of the vacuum container and the outside, an atmospheric pressure resistance structure called a spacer is interposed between the substrates. Generally, the spacer has a rectangular thin plate shape and is arranged while their edge portions are come into contact with both of the substrates in such a manner that its surface is parallel with the normal direction of the substrates.
A technical problem which the spacer has to satisfy is that not only an element as an atmospheric pressure resistance structure is necessary but also, in order to keep quality of a display image, it is necessary that its existence does not easily exercise an influence on a trajectory of an electron beam. Generally, a factor in which the spacer exerts an influence on neighboring emission electrons is understood as charging of the spacer. Several causes for the charging of the spacer can be mentioned. Fundamentally, it will be understood that a cause in which excess and deficiency of charges occurs due to transfer and reception of the electrons to/from an external region accompanied by incidence and re-emission of the electrons, so that the charging which exercise an influence on the trajectory of the electron occurs. As a technical solving method for the excess and deficiency of charges, there is a method of time-dependently obtaining an attenuating effect of the charge amount by applying electroconductivity to the spacer. As another solving method, there is such a technique that a secondary electron emission coefficient of the surface is set to a value which is not so larger than 1. Specifically speaking, there have been known a technique in which the secondary electron emission coefficient of a surface material which is applied is specified to a range of predetermined values, a technique in which a secondary electron emission amount is suppressed in a shape manner by providing a rough surface (concave and convex portions) for the spacer surface (refer to the specification of U.S. Pat. No. 5,939,822 and Japanese Patent Application Laid-Open No. 2000-311632 (U.S. Pat. No. 6,809,469)), and the like.
The charging which can be suppressed by the conventional method of forming a charge preventing film onto the spacer, forming the surface roughness, or the like is lightened within a non-selecting period of a display driving time. Such charging is instantaneously caused on the spacer surface by the electrons which penetrate into the spacer when the electron-emitting device is driven. That is, it is not a cumulative accumulation of the charges (hereinbelow, the charging charges in the short time and the charging phenomenon are called short time charging charges, short time charging, surface charging charges, or surface charging).
However, in the case where the concave and convex portions are formed on the spacer surface, although the short time charging of the spacer is suppressed, a change in beam spot position is cumulatively observed.